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    Higher temperature accelerates carbon cycling in a temperate montane forest without decreasing soil carbon stocks
    (Elsevier B.V., 2024-11-09) Siregar IH; Camps-Arbestain M; Wang T; Kirschbaum MUF; Kereszturi G; Palmer A
    Global warming is expected to accelerate the cycling of soil organic carbon (SOC) and the assimilation of new carbon, but the net effect of those counteracting accelerations and their ultimate effects on SOC are still uncertain. This hinders the prediction of long-term changes in biospheric carbon stocks and SOC-climate feedbacks. Here, we studied the long-term effect of temperature on carbon cycling across a 3.2 °C altitudinal temperature gradient in a temperate forest ecosystem in New Zealand. Across the gradient, soil respiration rates increased with increasing temperature from 9.0 to 10.4 tC ha−1 yr−1, but SOC stocks down to 85 cm depth also tended to increase, from 154 to 176 tC ha−1, albeit non-significantly (P = 0.06). This system was able to maintain higher soil respiration rates at higher temperatures without reducing SOC because the higher respiration rates were sustained by higher litterfall rates. Aboveground litterfall increased from 1.8 to 2.4 tC ha−1 yr−1 and estimated belowground C inputs increased from 7.2 to 8.0 tC ha−1 yr−1 along the temperature gradient. These higher fluxes were associated with significantly (P < 0.05) increased biomass at higher temperatures. As a direct measure of the effect of temperature on carbon cycling processes, we also calculated the turnover rate of forest litter which increased about 1.4-fold across the temperature gradient. This study demonstrates that higher temperatures along the thermal gradient increased plant carbon inputs through enhanced gross primary production, which counteracted SOC losses through temperature-enhanced soil respiration. These results suggest that temperature sensitivities of both plant carbon inputs and SOC losses must be considered for predicting SOC-climate feedbacks.
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    Studies on the dynamics of organic sulphur and carbon in pastoral and cropping soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 2000) Singh, Bhupinder-Pal
    Soil organic matter (SOM) can be depleted or regenerated by altering land management practices. Soil tests capable of reporting the size of dynamic SOM fractions may be useful for indicating the environmental cost of landuse and management practices. Information on the effect of land management practices on soil organic S content and turnover is scarce. This study evaluated the ability of a sequential chemical fractionation procedure to characterise changes in soil S and C organic fractions on a range of pasture and cropping soils with different management histories. The fractionation involved an initial extraction with ion exchange resins followed by dilute (0.1 M NaOH) and concentrated (1 M NaOH) alkali. In addition, recently rhizodeposited 14C (root+exudate derived) produced during a short-term (one week) 14CO2 pulse-labelling study of intact soil cores growing ryegrass/clover pastures, was used to trace the fate of root-derived C in both chemical and density fractionation procedures. In pasture and cropped topsoils, the major amounts of soil S and C were either extracted in 0.1 M NaOH (49-69% S and 38-48% C) or remained in the alkali-insoluble residual fraction (17-38% S and 46-53% C). These two fractions were more sensitive to change caused by different landuse and management practices than the resin and 1 M NaOH fractions. With a large amount of dynamic soil C remaining in the residual fraction it was concluded that increasing strengths of alkali were not capable of sequentially fractionating S and C in SOM into decreasingly labile fractions. The chemical fractionation allocated recent root and root-released 14C amongst all the fractions. Again, most root 14C appeared in the 0.1 M NaOH and residual fractions. Although small in amount, C of higher specific activity (more recently synthesised root C) was preferentially extracted by resin and 1 M NaOH extracts. Density separation was not capable of recovering recent root and root-released 14C in a single fraction. Root-derived 14C was distributed between light (mostly fibrous root debris) (42%) and heavy (organics attached to clay and silt) (45%) fractions. The dispersing reagent soluble fraction recovered <13% of the 14C. An anaerobic incubation and various acids and oxidising agents were tried, in order to recover a greater proportion of root and root-released 14C as a single identity. These were not very successful in either extracting or increasing the alkali solubility of the root C fraction. A 30% H2O2 pretreatment of soil plus roots, or hot 1 M HNO3 treatment of the residual fraction, were more efficient extractants of the root C fraction and should be investigated further to check their ability to better characterise soil organic S and C fractions with a change in management practices. The 14CO2 pulse labelling study of pasture swards showed a greater allocation of recently photo-assimilated 14C to the topsoil layer with a greater proportion of 14C recovered in roots than in the soil. An in situ soil solution sampling technique with mini Rhizon Soil Moisture samplersTM effectively monitored the rapid appearance of a 14CO2 pulse in soil water at various depths. A comparison of the 14CO2 pulse labelling study under light and dark conditions indicated that, in the light lysimeters, 14CO2 photo-assimilation/translocation/rhizosphere respiration was the main pathway for CO2 generation at various soil depths. In the dark lysimeters, 14CO2 diffusion was the main mechanism and 14C assimilation (either photo-assimilation or assimilation by chemolithotrophs in rhizosphere soil) was small. The 14CO2 activity in soil water from four soil depths of dark and light soil cores, and a CO2 diffusion model, were used to identify the 14CO2 contribution from rhizosphere respiration in the light lysimeters. A model was developed, but the unknown geometry of the air-filled pore space in the undisturbed soil cores made it impossible to precisely calculate the contribution made by root respiration to soil water 14CO2 activity.
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    Development of field techniques to predict soil carbon, soil nitrogen and root density from soil spectral reflectance : a thesis presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2009) Kusumo, Bambang Hari
    The objectives of this research were to develop and evaluate a field method for in situ measurement of soil properties using visible near-infrared reflectance spectroscopy (Vis-NIRS). A probe with an independent light source for acquiring soil reflectance spectra from soil cores was developed around an existing portable field spectrometer (ASD FieldSpecPro, Boulder, CO, USA; 350-2500 nm). Initial experiments tested the ability of the acquired spectra to predict plant root density, an important property in soil carbon dynamics. Reflectance spectra were acquired from soil containing ryegrass roots (Lolium multiflorum) grown in Allophanic and Fluvial Recent soils in a glasshouse pot trial. Differences in root density were created by differential nitrogen and phosphorus fertilization. Partial least squares regression (PLSR) was used to calibrate spectral data (pre-processed by smoothing and transforming spectra to the first derivative) against laboratory-measured root density data (wet-sieve technique). The calibration model successfully predicted root densities (r2 = 0.85, RPD = 2.63, RMSECV = 0.47 mg cm-3) observed in the pots to a moderate level of accuracy. This soil reflectance probe was then tested using a soil coring system to acquire reflectance spectra from two soils under pasture (0-60 mm soil depths) that had contrasting root densities. The PLSR calibration models for predicting root density were more accurate when soil samples from the two soils were separated rather than grouped. A more accurate prediction was found in Allophanic soils (r2 = 0.83, RPD = 2.44, RMSECV = 1.96 mg g-1) than in Fluvial Recent soils (r2 = 0.75, RPD = 1.98, RMSECV = 5.11 mg g-1). The Vis-NIRS technique was then modified slightly to work on a soil corer that could be used to measure root contents from deeper soil profiles (15- 600 mm depth) in arable land (90-day-old maize crop grown in Fluvial Recent soils). PLSR calibration models were constructed to predict the full range of maize root densities (r2 = 0.83, RPD = 2.42, RMSECV = 1.21 mg cm-3) and also soil carbon (C) and nitrogen (N) concentrations that had been determined in the laboratory (LECO FP- 2000 CNS Analyser; Leco Corp., St Joseph, MI, USA). Further studies concentrated on improving the Vis-NIRS technique for prediction of total C and N concentrations in differing soil types within different soil orders in the field. The soil coring method used in the maize studies was evaluated in permanent and recent pastoral soils (Pumice, Allophanic and Tephric Recent in the Taupo-Rotorua Volcanic Zone, North Island) with a wide range of soil organic matter contents resulting from different times (1-5 years) since conversion from forest soils. Without any sample preparation, other than the soil surface left after coring, it was possible to predict soil C and N concentrations with moderate success (C prediction r2 = 0.75, RMSEP = 1.23%, RPD = 1.97; N prediction r2 = 0.80, RMSEP = 0.10%, RPD = 2.15) using a technique of acquiring soil reflectance spectra from the horizontal cross-section of a soil core (H method). The soil probe was then modified to acquire spectra from the curved vertical wall of a soil core (V method), allowing the spectrometer’s field of view to increase to record the reflectance features of the whole soil sample taken for laboratory analysis. Improved predictions of soil C and N concentrations were achieved with the V method of spectral acquisition (C prediction r2 = 0.97, RMSECV = 0.21%, RPD = 5.80; N prediction r2 = 0.96, RMSECV = 0.02%, RPD = 5.17) compared to the H method (C prediction r2 = 0.95, RMSECV = 0.27%, RPD = 4.45; N prediction r2 = 0.94, RMSECV = 0.03%, RPD = 4.25). The V method was tested for temporal robustness by assessing its ability to predict soil C and N concentrations of Fluvial Recent soils under permanent pasture in different seasons. When principal component analysis (PCA) was used to ensure that the spectral dimensions (which were responsive to water content) of the data set used for developing the PLSR calibration model embraced those of the “unknown” soil samples, it was possible to predict soil C and N concentrations in “unknown” samples of widely different water contents (in May and November), with a high level of accuracy (C prediction r2 = 0.97, RMSEP = 0.36%, RPD = 3.43; N prediction r2 = 0.95, RMSEP = 0.03%, RPD = 3.44). This study indicates that Vis-NIRS has considerable potential for rapid in situ assessment of soil C, N and root density. The results demonstrate that field root densities in pastoral and arable soil can be predicted independently from total soil C, which will allow researchers to predict C sequestration from root production. The recommended “V” technique can be used to assess spatial and temporal variability of soil carbon and nitrogen within soil profiles and across the landscape. It can also be used to assess the rate of C sequestration and organic matter synthesis via root density prediction. It reduces the time, labour and cost of conventional soil analysis and root density measurement.